Field of the Invention
The present invention relates to personal identification systems, and more particularly to a pressure-sensitive signature verification pen and associated signal processing circuitry for use in such a system.
Description of the Prior Art
Personal identification systems are used where it is necessary to authenticate the identity of a person as a precondition to that person's taking certain actions or being granted specific privileges. A number of various personal characteristics or indicia uniquely associated with an individual are used alone or in combination to ascertain the identity of the individual. Among these characteristics or indicia are fingerprints, voice graphs, and signatures.
The use of a signature as the discriminating characteristic can be further analyzed in terms of several various measurable characteristics associated with the signature or act of signing. Those latter characteristics include the visual appearance of the signature, the pattern of pen pressure, the pattern of pen direction, and the pattern of pen velocity.
Perhaps the most frequently used characteristic associated with the signature is the pattern of pen pressure. This pattern may also be analyzed in terms of several associated sub-variables, including pressure maxima and minima, relative time of occurrence of the maxima and minima, average pressure, integral of the pressure over the signature period, and direct pressure pattern.
The measurement of pressure in these various forms may be achieved through the use of a pressure sensitive writing platen which either defines or forms a part of the writing surface. More often though, the pressure characteristics of the handwriter are sensed by a transducer associated with the writing instrument.
A common form of pressure sensing transducer is the strain gage. The strain gage is subjected to reactive load forces caused by the contact of the writing instrument with the writing surface. The strain gage may be mounted on a diaphragm that experiences mechanical deformation under the loading force, or may more directly receive the load force by being mounted contiguous with the cartridge in a ball point pen or similar type writing instrument.
There are certain identifiable problems associated with strain gages that reduce their effectiveness and desirability as a pressure sensing transducer in a signature verification pen. More specifically, strain gages are subject to variations in their resistance values with changes in temperature. They generally require a bridge network to develop an output signal, and the bridge network is subject to drift. Further, the strain gages must be carefully mounted in position to accurately sense the pressure, and moreover must be mounted in a manner that will allow them to withstand rugged use in the field.
Another form of pressure sensing transducer suggested by the prior art is a variable inductance. Generally, changes in reactive load force will be reflected as corresponding variations in the magnetic flux reluctance of an inductive winding. This form of transducer is necessarily complex in operation and costly to manufacture, making it of limited desirability for broad usage.
Still another form of pressure transducer is the use of an electrically conductive cartridge in the writing instrument that makes and breaks contact with a microswitch with the pressure or force pertubations transmitted through the cartridge. This type of transducer is inherently limited to producing a digital output signal. Moreover, the use of an electrically energized writing instrument is not well adapted to general purpose use in the field.
One type of pressure sensitive transducer which the art has not heretofore successfully adapted for use in a signature verification pen is the piezoelectric crystal transducer. A piezoelectric crystal produces a signal when mechanically deformed that is related to the time rate of change of the mechanical deformation. In this regard, it is analogous to a variable capacitor.
A number of reasons have been advanced against the use of a piezoelectric crystal as a pressure sensing element in a signature verification pen. Most prominent among those is the requirement for an integrated circuit to translate the piezoelectric crystal output signal into a direct representation of the pressure pattern. Moreover, integrator circuits are traditionally regarded as prone to drift, i.e. they require resetting with the introduction of each new signal.
However, piezoelectric crystals are inexpensive relative to other forms of transducer device, are easy to fabricate to the specific needs of the application, and are sufficiently durable to be able to withstand rugged use in the field.
A principal objective of the present invention is the adaptation of the piezoelectric crystal as a pressure transducer in a signature verification pen to realize the significant advantages of this device.